texts: What is the most distant observable White Dwarf known?

texts:
I attended a talk last week about Type 1a supernovae with the focus being on double-degenerate systems as potential progenitors for them. We were shown a graph of White Dwarf (WD) mass distribution like the one below (source), and it got me thinking, because even though they are very faint objects they are also very numerous in our galaxy.
So this led me to wonder what is the furthest known WD from Earth given that they are so faint but so numerous. I have looked for an answer online but the closest I got was a Type 1a progenitor at redshift 2 called "SN UDS10Wil" but this is not what I am looking for because this is not observable from Earth, it is just inferred from the supernova classification.


texts: Is the mass of Kuiper belt dominated by large objects?

texts: There is a problem in my example sheet trying to teach me distributions. My result is that Kuiper belt objects are dominated by large mass objects. Is this supported by observational evidence?


texts: Why don't the inner moons of Jupiter have tidally-induced volcanism?

texts: Volcanism on Io is caused by the fact that it is tidally heated. There are four moons that are closer to Jupiter than Io with higher eccentricities, yet they don't seem to have any volcanism at their surface. Why isn't the tidal heating enough to generate volcanism on any other moons of Jupiter?


texts: Could martian meteorites come from Olympus Mons?

texts: Could Olympus Mons have thrown ejecta out of Mars' orbit when it was active? Does it fit with the composition and dating of meteorites with martian origin?


texts: After doing research it said that nebulas are made of gas and dust. Are they actually gases and solids, or are they plasma?


texts: Are there many faint meteors that are too faint to see with the naked eye?

texts: I remember one time a while back looking at the night sky through night vision goggles.  One interesting thing I noticed is that there appeared to be many very faint meteors in the sky practically all the time.  I assume this is dust burning up in the atmosphere, or possibly an unusual effect produced by the night vision goggles themselves.  I more suspect that it is dust burning faintly in the atmosphere because it was seen only in the sky.  My main question is: Is space dust constantly entering the atmosphere as small meteors at a fairly high rate?  If I remember right, it was a few every second, which is much more frequent than normal "brighter" meteors, even during a meteor shower.


texts: At what annual rate are new exoplanets being recognized? How does it compare to new asteroids?

texts:
I just read the BBC News article Exoplanet tally set to pass 4,000 mark
. It makes me wonder about the annual rate  that new exoplanets being recognized, and how it compares to the rate of new asteroids.

I'm also wondering if there is any chance there will be a crossover in the rates in the next few years. TESS has found it's first exoplanet, and hopefully WFIRST will be on schedule as well.


texts: How should I interpret this result obtained using lomb scargle?

texts:
I used lomb scargle for periodic analysis.
The time interval used is constant at 500 seconds.

#My data (x=time, y=intensity values) is :
x= (
00:28,
00:37,
00:45,
00:53,
01:02,
01:10,
01:19,
01:27,
01:36,
01:45,
01:53,
02:01,
02:11,
02:19  )
y=(18264.682317  ,18720.708081  ,18476.409461  ,19592.046320
,19166.021184 , 18984.852405 , 20971.112057  ,20946.024653
21590.194169 , 21771.175308 , 21474.776018 , 20156.969939
21688.756080 , 22570.152358  )
--> The time interval of x data is equal to 500 seconds.
and lomb scargle code is:
frequency,power = LombScargle(x,y).autopower(nyquist_factor=1)
ax.plot(frequency,power,'-')
        #ax4.axvline(x=best_frequency)
ax.set_ylabel('(4-2) band R₁(2.5) Intensity - Lomb Scargle Power')

How should I interpret the lomb scargle results shown in the picture(below picture)?

Why does the maximum appear near zero(below picture)?


texts: What might cause a planet to have a significant tilt in their orbit?

texts:
Most of the planets in our solar system all orbit along the same plane, supposedly because of the almost flat dust clouds that formed the planets.

But some planets, such as the one featured below, have a significant tilt in their orbits that separates them from the normal plane of orbit for that star.

Image courtesy of Science Daily

What might cause a planet to have such a tilt? Is this the star capturing a wandering body into orbit or is this a phenomena caused by the formation of these systems?


texts: Observing cepheid variables in other galaxies

texts: If individual stars cannot be seen in other galaxies (excluding satellite galaxies of the Milky Way) then how can light from cepheids other galaxies be observed?


texts: What is the equation of state for a relativistic fluid/gas?

texts:
Say we have a relativistic fluid/gas, as we have in some astrophyical systems.

Now let us write:

$e$ - energy density in the fluid's rest frame.
$P$ - pressure in the fluid's rest frame.
$n$ - number density in the fluid's rest frame.
$m$ - mass of the particles.

I know that for the non-relativistic case we have:

$$e=nmc^2+\frac{1}{\hat{\gamma}-1}P$$

where $\hat{\gamma}$ is the adiabatic index. $\hat{\gamma}=1+\frac{2}{f}$ for a gas with $f$ degrees of freedom.

For the ultra-relativstic case we have:

$$e=3P$$

My question is what is $e(P,n)$ for a relativstic case (which is the general case of the 2 limits shown above)? I would also like to know how to derive it.

Is the following way the correct way to do it ? :

The number density of particles is:
$$n=\int_{0}^{\infty} n_p(p) dp $$

The pressure is:
$$P=\int_{0}^{\infty} \frac{1}{3} p v(p) n_p(p) dp $$

The energy density is:
$$e=\int_{0}^{\infty} \epsilon(p) n_p(p) dp $$

where:

$$n_p(p)= (2s+1)\frac{1}{ e^{({\epsilon(p)-\mu})/{k_B T}}+(-1)^{2s+1} } \frac{4\pi p^2}{h^3}$$

Here $s$ is the spin of the particles, for electrons $s=\frac{1}{2}$.

$$ \epsilon(p)=(m^2c^4+p^2c^2)^{\frac{1}{2}} $$

$$ v(p)= \frac{d\epsilon}{dp}=\frac{p}{m}\left(1+\left(\frac{p}{mc}\right)^2\right)^{-\frac{1}{2}} $$

From calculating the three integrals above we can finally obtain $e(P,n)$.

Can anyone confirm this is the proper way to do it, or am I missing something here?
It seems as if those integrals cannot be solved analytically - is this
true?
Perhaps in this case there is no explicit formula for $e(P,n)$?


texts: What is direction of Sun's axis of rotation relative to the Earth's orbital plane around the Sun?

texts:
The present northern direction to the nearest 0.5 degrees with degree zero being the start of January and measured counterclockwise (same as Earth's orbit relative to celestial north pole).

If the assumptions below do not allow a precision of 0.5 degrees then I imagine this question is unanswerable without specifying a specific date and time, in which case the specific date and time would be 1 January 2017, 00:00:00 hours.

Assumptions: (1) That the Earth's orbit around the sun is a perfect circle (I know it's very slightly elliptical but intuitively imagine that won't significantly affect the precision I need). (2) That the Sun's center is at the center of Earth's circular orbit (I know the Sun's center isn't exactly but don't imagine that will affect the accuracy I need either).

Note 1: I've found three general answers from three different sites, but, two of them appear to me to conflict and the other is too imprecise.

Note 2: This is my first post to this site and I did not plow through the hundreds of questions related to these tags. So if this question has already
been answered I apologize and please be merciful and point me in right direction.

Thank you


texts: What is the acceleration for the expansion of the Universe?

texts:
I've found that the speed of expansion is about $70\frac{km}{s\cdot 10^{6}pc}$.

But what is the acceleration of expansion in terms of $\frac{km}{s^{2}\cdot 10^{6}pc}$ ?

I want to know how fast this value of "70" is changing, ie what is $\mathrm{d}H_0/\mathrm{d}t$?


texts: Cooling of astrophotography camera sensor chip

texts:
For astrophotography using 'medium size' land telescope at good location,

How many time longer exposure, in view of boosting the system's ability to photograph even darker objects, can one gets from cooling down digital camera sensor chip?
Is there a 'sweet spot' temperature where most thermal related noise is suppressed and further lowering of temperature yields litter extra benefit.
What is the ball park temperature for low (amateur), medium and high (professional) grade cooled camera?

Web search shows that Peltier electric cooled camera works at around "minus a few tens degrees centigrade". Does the really professional one cool to more extreme level, somewhere close to liquid nitrogen temperature?

Does CCD, CMOS or other sensor have different 'sweet spots' temperature?


texts: The Earth is differentiated, as it has a distinguishable core, mantle and crust with different compositions and densities. Are all the other planets differentiated as well?


texts: Could rocks from Earth have reached the Kuiper belt, or Neptune at least? If so, how?

texts:
This answer  (currently edit 3) to How certain are we that we have not sent life to other planets/moons? begins:

First of all, rocks from Earth are probably just about everywhere in the Solar System. One simple example is this rock found on the Moon. A number of pieces of Mars have been found on Earth, and if that has happened, no doubt there are Earth rocks on Mars. If Earth life can survive a vacuum it has probably gotten to everywhere in the Solar System anyways. (emphasis added)

I started to write the following as a comment:

"...rocks from Earth are probably just about everywhere in the Solar System" is quite a statement, and  ...from which point they will travel throughout the solar system. requires energy. What mechanism accelerates a rock from Earth to the outer solar-system? Mars is not really a distant object compared to the size of the solar system. New Horizons is in the Kuiper belt already. Is it even remotely plausible to suggest that Earth rocks can become KBOs?

Question: Can rocks from Earth have reached the Kuiper belt or Neptune at least? If so, how?
I'll even allow a solid particle of proper space-dust to serve as proxy for the rock for the purposes of this question, even sub-micron in size if necessary, but not simply an atom or a molecule. We know from answers to What is the origin of the dust near the sun? that dust is surprisingly (to me at least) mobile in the solar system due in part to its large surface to mass ratio and interaction with static EM fields and photons for example.


texts: What is the relation between the saros (18 years, 11 days, 8 hours) and the period of lunar nodal precession (18.6 years)?

texts: I am wondering. I am sure that they are related. How can we derive one from the other?


texts: Standard practices and models for generating the ephemerides of the solar system

texts:
I am familiar with the theory on generating the positions of the members of the solar system, but have been unable to find out the standard practices and methods of professional astronomers in acquiring this information. The basic methods of generating an ephemeris of the solar system can be found on the Wikipedia. Also, I have the book Astronomical Algorithms by Jean Meeus.

How do practicing astronomers do it though? Are there standard programs or software that everyone uses, or do they access servers at some international body that returns positions at a given time, or does each astronomer have his own custom program that he made? What is the normal way an astronomer gets planetary positions?


texts: As viewed from Mars, what are Jupiter's and Saturn's maximum brightness in apparent magnitude?

texts:
From Earth, Jupiter's maximum brightness is -2.94 and Saturn's is -0.24. But what about from Mars? They should be brighter, but by how much?

There are equations at the obvious wiki entry but I'm not sure I understand them. Plus I'm afraid to attempt this calculation myself because I've heard that this sort of thing does not follow the inverse square law. They're reflecting light (not generating it), and I read somewhere that it should follow an inverse 4th power law because of that. I don't see any 4th powers in the apparent magnitude equations.


texts: If Earth suddenly stops rotating, will be thrown at 465 m/s?

texts: If Earth suddenly stops rotating, will be thrown at 465 m/s ?


texts:
From watching a video on youtube, it states that Dark Matter is 6 times more abundant than known matter. I was wondering if it was possible to have hydrogen clouds between the universes galaxies. The density would need to be less than inside universes galaxies (since star formation would be possible).

Is it possible that hydrogen clouds be a major component of Dark Matter?


texts: I just watch an interesting video about Moonphase watch. mmm... but why do I need to know about moonphase anyway, I just can't think of any practice for the moonphase indicator on the watch?


texts: Could a space radio telescope fill multiple purposes?

texts:
Dream for a while and imagine that there will be a flagship+ mission within a decade to put a large radio telescope in space:

What kind of different tasks could the same radio equipment feasibly be used for?
How do the requirements for frequency sensitivity, electric power supply, dish size, and other radio technology (such as applicability of phased array?) and what else vary most importantly between such purposes?

For example, could one and the same big space radio telescope in the Asteroid Belt be used for any, some or all of the below, what would the synergies and contradictions be:

a) Radio astronomy on its own.

b) Interferometry together with a telescope on Earth to create an interplanetary baseline for huge resolution, like the spektr-R does cis-lunar today, AFAIK.

c) Radar characterization of the orbit, shape, size et cetera of any large enough asteroid in its current section of the asteroid belt.

d) Ground penetrating radar to characterize the interior composition of near flyby asteroids.

e) Communication with Earth so powerful that it need not engage the Deep Space Network (DSN), but can work with smaller Earth based radio antenna.

f) Extending the DSN by working as a relay between Earth and outer space probes, or between Earth and the backside of Mars or when Mars is in apposition behind the Sun from Earth.


texts: How to calculate distance from object to point on celestial sphere


texts: How to improve efficiency in periodicity detection?

texts:
Our data set has $10^4$ data points, but has a long baseline and many gaps.

If we bin the data, there would be $10^8$ data points ($[t,\rm {value}]$), but only about 1% are non-zero values.

How to improve the detection efficiency?

Is a multi-threading way possible (especially for Lomb-Scargle)?

For example, if I use LombScargle in astropy.stats,

freq, power = LombScargle.autopower(minimum_frequency=0.5, maximum_frequency=1.5, normalization='standard')

there are two problems:

It is slow and easy to get memoryError.
It can't utilize the computing power as much as possible.

So my question is about the efficiency. For the data set above, what is the best method?


texts: What evidence is there of Earth-Like internal features of Europa?

texts:
This question is inspired by "What are the Earth-like features of Titan?".  According to NASA's Europa overview, Europa is believed to have an iron core, rocky mantle and an ocean under the frozen surface.

Does the tidal heating of Europa's interior by Jupiter make it a smaller analogue for Earth?


texts: Why the Milky Way appears to be arching across the sky?

texts:
The Milky Way galaxy is disc-shaped and earth is in its plane:
 (source)

In the following image the milky way appears to be arcing across the sky:
 (source)

This looks like an out of plane view of the milky way... How is it possible? Why an arch and not a thick straight line?


texts:
News has come up lately about the impending approach of asteroid 2017 VR12 and how it might be visible to some backyard telescopes. However I've read conflicting or vague reports on what kinds of telescopes will be powerful enough to spot it.

I've got a modest but still decent little Celestron NexStar 4 SE, which is a 4" Schmidt-Cassegrain. Can anyone say for sure whether that has the power to see the asteroid? Currently I just have the stock eyepiece. Would I need to get a better one to help out?


texts: What does the term alien planet actually refer to?

texts: This article left me wondering what do they mean by the term alien planet? Aren't all planets simply alien by default, or do they mean planets outside our solar system are alien? Do they mean there is alien life there? Is this a a new keyword the astronomy community is using to generate more buzz and get more attention to their articles?


texts: If nothing travels at the speed of light, except light, how can a black hole also pull light into itself?

texts: Is it because it has super massive gravity and energy? Or is it some sort of an anti-light component in it because it used to be a star?


texts: The most feasible hypothesis about Theia

texts:
The formation of the Moon had many hypothesis over the years, most of them involving a Mars-sized body called Theia, which presumably collided with the Earth. But even within the Theia hypothesis, is not very clear the conditions of that collision. So, what is the most plausible hypothesis about Theia today?

Theia fully disintegrates and the remnants debris form the Moon in a head-on collision(the Big Impact hypothesis)
Theia-Earth suffer a glazing collision and Theia manages to escape to space, but some of the remnants of the collision form the Moon (Reufer, 2012)
Other hypothesis, such a double Theia system

All the hypothesis have problems, but to me the most believable is 2, because it solves the majority of the problems of the other ones, although there's not much consensus on that one, the only who defends it is Reufer.

For example, some research shows that only 2% of Theia actually contributed to the formation of the Moon (I don't remember the reference), so 1 doesn't seem too much feasible because according to that one Theia fully disintegrates.


texts:
Would like to understand the visibility of the moon from an astronomical point of view of view.
Is it possible for a waxing crescent moon to be visible in some countries while not being present in others?


texts: Site to see direction to the Voyager by entering time and coordinates of my position on the Earth?

texts: Where is Voyager relative to me right now in realtime?


texts: What is the lifetime of an x-ray binary?

texts: What would be the typical lifetime of an x-ray binary? I have trouble finding a reference.


texts:
Do other universes like ours exist?
If they exist, how do we know that they exist when we have even not seen the ends of our own universe?


texts: Magnetic fields of peculiar HgMn A type stars

texts: Do HgMn (peculiar A type) stars really possess global magnetic fields? See, for example, this paper by Hubrig et al. from 2012.


texts: How are the solar flares from May 23, 2021 extraordinary?

texts:
The Space Weather News for May 23, 2021 reads (with some highlighting by me)

Yesterday, the sun produced a sequence of solar flares unlike anything we've seen in years movie. Earth-orbiting satellites detected a dozen explosions in the magnetic canopy of sunspot AR2824. One of them emitted a radio burst so strong, it drowned out static from lightning storms on Earth and was recorded at midnight by listening stations in the Arctic. NOAA forecast models suggest that a CME might hit Earth's magnetic field on May 26th.

As non-expert of that field, I have a couple of comprehension questions:

What is a "magnetic canopy of a sunspot"? Is there a definition for that, or does it just mean "magnetic flux tube coming originating at the sunspot"?
Why is that solar flare so spectacular? Which parameters are the most extreme ones? Where could I find more on the statistics of those parameters? What exactly does "in years" mean here? How rare are which parameters of the May-23-flare?
How are sunspots named? A quick search showed that AR2824 seems to be around for quite a while, but I did find anything about whether that is a simple counter, or there are different prefixes, other than AR.
It is said that the "CME might hit Earth's magnetic field", which is probably not same as "it will hit Earth", or is it?  Could somebody please guide me how this is read from the (above linked) NOAA forecast model graphs? Do these models give a probablity of a CME reaching Earth? How likely is it that this CME might disturb any satellite?

In short: I would appreciate a quantified and expanded version of the quoted news, enriched with some definitions for the non-experts.
References and related questions

Royal Observatory of Belgium's Sunspot Number
Sunspot Magnetic Fields Handout
What are the main differences between solar flares and coronal mass ejections?
Solar flares/CME dangers to life?


texts: Shouldn't all moon orbits be inherently unstable?

texts:
A hypothetical simulation starts with a planet called Earth in a perfectly circular orbit around a star called Sol. There is also a moon called Luna in a perfectly circular orbit around Earth.

Question:
Can Luna's orbit remain perfectly circular or does it become elliptical over time? At what point does it become stable? Never?

Thinking:
The distance between Sol and Luna is not constant, it fluctuates as Luna orbits Earth. Therefore the pull sunward also fluctuates. Shouldn't this cause all moon orbits to eventually deteriorate into longer and longer elliptical orbits until they collide with the planet they orbit?


texts: Is it possible to calculate the focal length of a refactor I built?

texts:
I am trying to build a very low cost refractor telescope. It's for a project, so I need to be able to provide "data" about the telescope. But it seems I can't calculate anything (magnification, to be more specific) without knowing the focal length of my telescope. However, because I built it myself, I do not know what my focal length is. How can I find that out, or how can I calculate the magnification?

On another note, what is the best combination for the best magnification? I am not looking for anything great, just the best magnification possible.

I can have a 50mm lens with a fl of 50cm, 30cm, and 20cm and a 38mm lens with each of the same focal lengths. Which is the best pair?


texts:
How do asteroids, comets and other stuff stay together when colliding in the empty vacuum of space, when they are made up of non-sticky substances like rock?

My understanding is that the force of gravity from small objects is very weak and when travelling at speed I don’t see how they join together.


texts: On Augustus' Actual Prescription to Restore the Julian Calendar to Accuracy

texts:
I hope that I may ask this question here as I have seen some favorably received questions related to the Julian calendar on this site.
From James Evans' book, ``The History and Practice of Ancient Astronomy,'' is the following snippet (pg. 165):

In praticular, it indicates that ``to bring the [Julian] calendar back into step with the original plan, Augustus decreed in 8 B.C. that all intercalations be omitted until A.D. 8.''
I know the Romans were ``inclusive'' counters---so may I interpret this as meaning that leap years did not resume until 12 A.D.?
Another source: https://www.tondering.dk/claus/cal/julian.php specifically states: ``There were no leap years between 9 BC and AD 8 (or, according to some authorities, between 12 BC and Ad 4).''
QUESTION: Does anyone know for certain if A.D. 8 was a leap year or not; and would you please provide an authoritative reference if you know of one?
Thank you.
I have also read elsewhere: ``


texts: Sun's apparent motion above the arctic circle during summer solstice

texts:
I'm having difficulty with an apparent contradiction about the sun's motion during the summer solstice above the arctic circle.

The YouTube video The North Pole Entire Summer of never ending sun Summer Solstice High speed camera and solar motion models show the sun to move clockwise around the horizon for the full 360 degrees during the summer solstice.
However, when I walk in a circle counterclockwise, to represent the earth's rotation, and focus upon a fixed object outside the circle to represent the sun, I get a 180 degree motion, from "east to west", and then the sun's apparent motion returns again from west to east, not the full 360 degrees which everyone reports is the actual phenomenon.

Can someone help me to understand my dilemma?


texts: Why do we only see the clouds directly infront of the moon glow from moon light?

texts: Surly the moon is not projecting a direct beam of light which only lights up the small area of clouds around the moon. What phenomenon is happening that only allows us to see that center focus glow and not all the clouds glowing?


texts: Meade LX200GPS electronic RA movement very slow

texts:
I use a Meade 200 LX GPS.

Last night between viewings it was placed in Sleep mode.
On return the scope was making noises as if the motor was running. The scope was stationary, but had moved to what looked like park position.

I pressed MODE and then tried to locate objects using GOTO, but it did not move appropriately either from where it was, or as if from the previous sleep position.

I then switched everything off and 'started again'...several times.

I received a couple of strange messages including that I had changed the location of the scope (it had not moved) and that the batteries were low (it is connected to the mains, not running on batteries).

I then decided to leave it overnight and retry in the morning.

This morning I started from scratch again and successfully carried out a 2-star alignment, and the scope moved at normal finding speed to the star locations. However, when centring the stars electronically the RA movement was VERY SLOW no matter what speed setting was set. Dec speed is fine!

The scope moves to GOTO targets at normal speed, but electronic control of RA is still VERY VERY SLOW.

Could you please advise what may be the cause of this problem and best options for remediation.

FYI we have two hand box controllers and the problem occurs with both. The hand box is not the fault!


texts: Is it possible for particles to be accelerated FTL inside BH travelling towards the singularity?

texts:
Maybe just maybe it is possible for a particle to travel FLT inside of black hole  because of the singularity gravitational force

This could be due to the fabric of space it self falling towards the singularity FTL beyond the event horizon. So technically
A particle can go FTL moving with space itself not not moving through it, and this would break any laws of relativity

But if I'm wrong please tell me.


texts:
I was reading this article
 and it says the following:

  Researchers measured the mass of the Milky Way and found that our galaxy is approximately half the weight of a neighbouring galaxy known as Andromeda which has a similar structure to our own.

So I was thinking, if we consider a galaxy neighbor to us, then our galaxy should be having an endpoint and the neighboring galaxy should be having a start point. So how do we know which is the starting and ending point of a galaxy, and how do we calculate it?


texts: Calculating length of summer solstice day using tilt and latitude

texts:
I want to be able to predict how long the summer solstice lasts at any part of world given the maximum tilt angle $φ = 0.4101524 \; \mathrm{rad}$ and the latitude $θ \; \mathrm{rad}$. I specifically want to do this using a geometric approach. The problem is, I never had experience with spherical trig before, and the values I got are somewhat off from their real values. So I want to know if that's because I didn't do the math correctly, and/or because of physical reasons (e.g. the Earth isn't perfectly spherical, the tilt changes slightly etc.).
Here is how I did it:

Let the Earth be represented as a perfect sphere of radius $R$ moving at a constant angular velocity. Let the Earth's rotational axis be in the z direction, and the axis of the circle forming the dark/light boundary be tilted by angle $φ$ relative to the Earth's axis. During the summer solstice, our location of interest (say, in the northern hemisphere) will trace out a circular path of radius $r < R$ perpendicular to the z-axis. These three circles, $C_{\text{rot}}$, $C_{\text{bound}}$ and $C_{\text{path}}$ create a spherical triangle. Using the arc length $s$ of this triangle along $C_{\text{path}}$, we can calculate the length of the day in hours using: $$24 \times \frac{πr + 2s}{2πr}$$

The point where $C_{\text{rot}}$ and $C_{\text{path}}$ meet is located at $$P(\text{rot/path}) = (R\cosθ, 0, R\sinθ)$$

To find the point where $C_{\text{bound}}$ and $C_{\text{path}}$ meet, we first observe that $$y = \sqrt{R^2 - x^2}\sinφ$$ and $$z = \sqrt{R^2 - x^2}\cosφ$$
Which we can solve for $x$ by setting $z$ equal to the $z$ from the previous point
$$x = R\sqrt{1 - \frac{\sin^2θ}{\cos^2φ}}$$
Plugging this back into $y$ and $z$, we finally get
$$P(\text{bound/path}) = \left (R\sqrt{1 - \frac{\sin^2θ}{\cos^2φ}},R\sinθ\tanφ,R\sinθ \right )$$
Now these two points are on the same plane in the z-axis, which is the plane of $P_{\text{path}}$. The distance $d$ between these points is
$$d = \sqrt{\left (R\sqrt{1 - \frac{\sin^2θ}{\cos^2φ}} - R\cosθ \right)^2 + R^2\sinθ^2\tan^2φ}$$
From this we can get $s$ by using the arc length formula:
$$s = 2\arcsin \left (\frac{d}{2r} \right )r$$
I used Mathematica to compare the calculated and expected values in 2017:

New York: $θ = 0.7105724077$; length (calculated) = 14 hours 56 minutes; length (expected) = 15 hours, 5 minutes

London: $θ = 0.8989737191$; length (calculated) = 14 hours 25 minutes; length (expected) = 16 hours, 38 minutes

As you can see, there is a noticeable difference (10+ minutes).


texts: Orbital velocity of a planet - why is my calculation off by about 10%?

texts:
I am not sure if I am doing something wrong, or misunderstanding Reider and Kenworthy (2016).

I'm just trying to reproduce the orbital velocities listed in Table 1. The second paragraph of Section II list a mass of the primary and semi-major axis for the planet's orbit of 0.9 solar mass and 5.0 AU. From the table the mass of the planet ranges from 20 to 100 Jupiters, which is actually quite sizable, but I'll start without using the reduced mass.

The numerical values I'm using:

$$GM_{\odot}=\text{1.327E+20} \ \mathrm{m^3  kg^{-2}}$$
$$GM=0.9GM_{\odot}$$
$$\epsilon=0.65$$
$$1 \ \mathrm{AU} = \text{1.496E+11} \ \mathrm{m}$$
$$a=5.0 \ \mathrm{AU} \ = \text{7.480E+11} \ \mathrm{m}$$

The formulae I'm using:

$$r_{\text{peri}}=a(1-\epsilon)$$

$$v^2=GM(2/r-1/a)$$

$$v_{\text{peri}}=\sqrt{GM(2/r_{\text{peri}}-1/a)}$$

I get:

$$r_{\text{peri}}=\text{2.618E+11} \ \mathrm{m}$$

$$v_{\text{peri}}=\text{2.744E+4} \ \mathrm{m/s}$$

which is  $27.44 \ \mathrm{km/s}$. But for $\epsilon=0.65$ the table below shows $29.5 \pm 0.4 \ \mathrm{km/s}$. Close but not really close enough, it's off by almost 10%.

If the mass of the planet (which is quite large) was considered, then the table would have to list a wider range of velocities, wouldn't it?


texts: in current models of the Big Bang, what happens to the products of baryon annihilation?

texts:
In current thinking about the Big Bang, the baryogenesis phase involves CP/CPT symmetry violation. There is an excess (says Wikipedia) of perhaps 1 in $10^{10}$ baryons over antibaryons, the majority annihilate and some matter is left over.
That's all fine so far except - the energy output (gamma rays?) of annihilating something like $2 × 10^{10}$ times as much baryonic  matter/antimatter as remained in the universe after baryogenesis, sounds like it should have lit up the early universe. But the models seem to skip over this, and the next thing we hear is the classic mass-energy distribution of around 63% dark energy, 23% dark matter, 4% baryonic matter.
Where did the vast production of high energy photon pairs (I guess?) from the annihilation phase, end up? What happens to them, in this model, at and after annihilation? Thinking of background neutrino/background microwave from other singular large-scale "creation-of-afterglow" Big Bang events, where are they now?


texts: How do ASKAP's focal plane phased array feeds interact with the entire array phasing?

texts:
Sky & Telescope's ASKAP Joins the Hunt for Mysterious Bursts says:

  A new telescope, the Australian Square Kilometre Array Pathfinder (ASKAP), has joined the search for energetic and elusive fast radio bursts. And in just a few days of looking, it’s already had success!

Later on, the article goes into some detail about ASKAP:

  A new player is now on the scene, however, and it’s already had huge success. ASKAP is a wide-field radio telescope made up of an array of 12-meter antennas. Using phased-array-feed technology, ASKAP is able to instantaneously observe an effective area of 160 square degrees — an enormous field compared to Parkes’ 0.6 square degrees! This capability significantly increases our chances of being able to detect fast radio bursts.

The article links to The Detection of an Extremely Bright Fast Radio Burst in a Phased Array Feed Survey which is not paywalled and contains some discussion of the array.

This sounds really interesting! It is a phased array of dishes, but at the focal plane of each dish there is also a phased array of receivers.

This is different than just adding an array of feed horns and independent receivers as I've discussed in my surprisingly robustly unanswered question What is the highest granularity focal-plane array on a dish radio telescope? Or is this the ONLY ONE? where receivers measure intensity as pixels, in a similar way to a camera (thus the counterintuitively placed astrophotography tag there).

Here, each focal plane is equipped with a true phased array, where if I understand correctly, phase relationships between elements in the focal plane are maintained and recorded.

According to Wikipedia and CSIRO's Australian Square Kilometre Array
Pathfinder – Fast Facts there are 36 separate 12-meter dishes in the entire array, and the focal plane of each dish is equipped with a 188 element focal-plane phased-array. 188 $\times$ 36 = 6768 channels, not counting polarizations because I don't know if they are separate channels in this case.

That would require quite a hefty correlater if the analysis was done flat, without any heierarchy in the calculation. This is a pathfinder for the SKA so pushing limits is important, but I'd like to know how this large number of signals is handled. Is there pre-processing?

Question: How do the phased array feeds on each dish of the ASKAP interact with the entire array phasing?

I'm not sure of the "focal plane array" is located precisely at the focal plane or not. If it were, then a given source in the sky would produce signals in only one, or a small group of receivers, and not the whole focal plane array! One normally uses a phased array instead of an imaging system so there must be something more interesting going on here, perhaps only nearest and next-nearest neighbor correlations within the focal plane array but full correlation between the dishes?

How does this thing really work?

below x2: Cropped from CSIRO ScienceImage 2161 Close up of a radio astronomy telescope with several more in the background.


texts: How might WFIRST help us better understand dark energy?

texts:
In the video linked in the Space.com article What Would It Mean for Astronomers If the WFIRST Space Telescope Is Killed?, (available in YouTube as WFIRST: The Best of Both Worlds) after 02:00 Caltech cosmologist Yun Wang says:

  WFIRST will allow us to potentially make ground-breaking discoveries, finding out what dark energy is. So this will tell us if dark energy is an unknown form of energy, or if its a modification of general relativity.

Without going too deep in to GR, how might that happen?

Question: What kinds of WFIRST observations  and interpretations would be used to potentially address the nature of dark energy?

Based on comments, I'll reclassify the following as an afterthought:

Is this in any way related to news about a recent observational non-exclusion of Erik Verlinde's theories (the guy who's "Trying to kill dark matter"), discussed in Margot Brouwer's 2016 video Verlindes new theory first tested with weak gravitational lensing about the VLT Survey Telescope kilo-degree survey results (see also Cosmos Magazine's New theory of ‘dark gravity’ passes first test, but Einstein’s still on top, or am I mixing apples and oranges?


texts: Which spiral arm of the Milky Way is Kepler-62 in?

texts:
I am trying to find out which spiral arm of the Milky Way the star Kepler-62 and its planetary system are in. All I can find by Google searching is that it is part of the Lyra constellation, but I don't know if that entire constellation is within 1 spiral arm or not, nor which arm it is in.

Is Kepler-62 in the Carina–Sagittarius Arm, or in the Orion Arm, or...?

The only map picture I can find that is somewhat related is this (I assume that the yellow light area is the area in which Kepler-62 was sighted by the Kepler spacecraft): http://upload.wikimedia.org/wikipedia/commons/b/be/LombergA1024.jpg

Thank you!


texts: What was the "brilliant new star in Aquila" on June 8, 1918, just after the solar eclipse?

texts:
This great answer about the US Naval Observatory's $3,500 expedition to Baker City Oregon to observe the June 8, 1918 total solar eclipse links to the January 1919 Popular Astronomy article about the expedition. The article notes on page 6 the following coincidental observation:

  An interesting occurrence was the appearance of the brilliant new star in Aquila. This was seen by Mr. Peters and Mr. Conrad on the night of June 8 and the discovery was communicated by telegram to various observatories in the country. It had however been discovered earlier on the same night by other astronomers in the east.

What is the story behind this bright new star in Aquila? What was it? Who discovered it first?


texts: What would theoretically be dissolved in the interior oceans of icy moons and planets?

texts:
I know we have some indication of "what's in the water" from the plumes of Enceladus, but would other places be the same. For instance, could the deep liquid water of Titan contain dissolved methane gas?
I'm mainly contemplating what possible "solutions" would be beneficial to some form of microbiology.


texts: Is the eventual heat death of the universe due to the expansion of the universe?

texts:
I am reading about the accelerating expansion of the universe and something struck me. The observation that things further away were accelerating away faster and that early observations made it look as though we were situated at the center of the universe.

Specifically, it seems as though the universe itself is expanding and not that the things within the universe were moving away from each other. Instead it seems as though the expansion itself is creating more space between the contents.

Recently I also saw an amazing video on the eventual death of everything in the universe. This seems weird to me because as I understand it energy and matter don't go away they just change.

And it got me thinking, what if this eventual heat death of the universe is simply this expansion eventually ripping everything apart until every particle simply is to far apart to interact.

Basically, as things progress the expansion will affect the contents of the universe even down to the particle level.

Is this the case? Or am I misunderstanding this subject?


texts: What will New Horizons be able to say about a magnetic field at Pluto?

texts:
Does NASA's New Horizons flyby probe to Pluto have instruments to measure Pluto's (or Charon's) magnetic field, if any?

I'm a bit confused by the Wikipedia article which says that New Horizons, during its flyby of Jupiter, on one hand:

  Travelling through the planet's magnetosphere New Horizons collected
  valuable particle readings.[45] "Bubbles" of plasma which are believed
  to be formed from material ejected by the moon Io were noticed in the
  magnetotail.[47]

But then on the other hand:

  An objective to measure any magnetic field of Pluto was dropped. A
  magnetometer instrument could not be implemented within a reasonable
  mass budget and schedule, and SWAP and PEPSSI could do an indirect job
  detecting some magnetic field around Pluto.

And how true is this story which I read or heard somewhere? That when New Horizons was being built and launched, no one really believed that Pluto could have a magnetic field. But since then planetary science has advanced and it is now plausible that it might have a magnetic field, because of tidal forces between Pluto and Charon which might make both objects somewhat fluid and possibly generating magnetic fields.


texts: How is the size of a distant planet determined?

texts: How is an object's size determined from other solar systems than our own? How are new large bodies being found in our solar system currently?


texts: How to derive the depth in my image (limit Magnitude )?

texts:
I saw that some survey said that their depth is 27.5 magnitudes.
May I ask that if I have a dataset with magnitude and their errors, for example I have J band j_m, j_cmsig, j_msigcom, j_snr from Sextractor.
How can I derive the 3 sigma magnitude depth in this fields to compare with the other survey.


texts: It moved in a straight line with sparkling track.What could it be?

texts: Today I saw something that passed like a jet. But am sure it's not so because jet will leave a long track of its passage. It moved in a straight line with sparkling track and what I saw was observed even from  200kms away from my destination. What could it be?


texts: Is stacking welder's glasses a safe way to watch at the eclipse?

texts:
You can find in many place on the Internet that welder's glass #14 is good for looking at an eclipse. Tomorrow (March, 20th 2015 at 10:45 CET) there's a solar eclipse and yesterday I could only find glasses #11 and #9. I tried briefly this morning, and combining a #11 and a #9 was giving me a good view of the sun. Using 2 #11 gave a too dark image. Now, I read different opinions on the internet.

Against (Perkins Observatory):
http://perkins.owu.edu/solar_viewing_safety.htm

  Be careful that you use the right kind of glass! Welder's glass is
  numbered from 1 to 14 with 14 being the darkest. It is only number 14
  glass that is dark enough for solar viewing! And NO STACKING! A pair
  of number 7's or a 10 and a 4 together DO NOT have the same protection
  as a single piece of number 14 (see unsafe methods for more details).

Favorable (Royal Astronomical Society of Canada):
https://www.rasc.ca/tov/safety

  If SN14 filter is not available, it is possible to combine lower shade
  numbers to get roughly the same level of eye protection from solar
  radiation, e.g. combining SN 6 and SN 8 filters. However the image
  quality may be considerably poorer than that seen through the single
  SN14 filter

I could not find a table or something explaining which kind of protection gives each number; according to the Canadian website, the only concern is about how much infrared light goes through, ultraviolet does not seem to be a problem in almost any case (I was surprised to read that).

Note: It's not my intention to open here the discussion on what could be other safe methods to watch the eclipse, this is well explained everywhere around. I read too late about the eclipse to order specific glasses.

Edit:
One of the answers here report the following formula: (more insight at this link)

  13 or darker is safe enough. Also, you CAN add up welding glass, using
  the formula S(sum) = S1 + S2 -1. S(sum) should be greater than or
  equal to 13


texts: Today in the tech age we have many communication technologies cell tower for cell  phones,  wifi, nuclear subs have some classified method of communication,  etc.  What communication technology is being used if a satellite probe is sent past Jupiter & transmits a signal back to scientists to monitor progress or review high definition detailed images?


texts: Are there upper or lower mass limits for black holes?

texts:
Natarajan & Treister (2008) describes a practical upper limit for black hole masses at $\sim 10^{10} M_\odot$. This is all due to the black hole's interactions with nearby matter.

However, is there a theoretical upper mass limit for black holes in general relativity? More specifically, do any solutions make note of this? Would this depend on whether the black hole described is eternal or time-variant, static or spinning, charged or uncharged, etc.?

Similarly, do any metrics make note of lower mass limits? Would it be possible for a black hole with the mass of an electron to exist (at any point in time, putting aside Hawking radiation)?


texts: Is nucleosynthesis responsible for the expansion of the universe?

texts:
Is it just a coincidence that the two major expansionary periods occur close to periods of nucleosynthesis?

Big Bang nucleosynthesis occurred very close to the inflationary period.
Supernova and stellar nucleosynthesis created heavier elements as the stars died, and the universe underwent increased acceleration.

Further thoughts:

The metaphor for the expansion of the universe is often described as an inflating balloon, but could it be equally valid to think of expansion as matter falling into its own gravity well, away from infinity minus other matter within its gravity field. The effect being that matter radiates space itself as gravity pulls itself away from infinity. Just as heavier elements create a stronger gravity field, heavier elements would accelerate away from infinity minus other matter within its gravity field, at a faster rate.

Eventually, on a cosmic scale, atomic decay would release the confined energy and the energy would return to infinity, creating the appearance of a contracting universe that restarts the Big Bang.  If this idea is reality it would create a geodesically complete cosmology.


texts: Are the planets in our solar system in free fall?

texts: If I’m correct, the International Space Station is in free fall, but is moving fast enough to keep it’s position. Assuming that is true, does that mean that some/all planets orbiting the Sun are in free fall of the Sun?


texts: Mars is dark, surrounding is bright, what am I doing wrong?

texts: I just got a Celestron 8 and I'm a complete rookie: set up scope early so it would acclimate to the temperature. No difficulty locating Mars, but I see a dark planet with its circumference slightly illuminated and the night sky around it is bright? (scope in good dark location and nice dark sky) No amount of fine tuning makes a difference. What am I doing wrong?


texts: Why do exoplanets a little more massive than Earth, often appear to have lower densities than Earth from transit measurements? Assuming that that they are terrestrial planets, what four(I am also nore sure if it is four) chemical elements make up the bulk of their mass?


texts:
I heard a theory (possibly from a crackpot) that there was an event in our solar system where two objects collided and created the asteroid belt, the orbits of some of the planets were altered, and that this event took place late enough for homo sapiens to have referenced observations made during the previous orbit in their oral traditions. (Mars parsing close enough to Earth for people to observe its moons)

Is any of this possible given readily observable evidence?
Are asteroid belts ever created in this way?


texts: What are some formulas that are associated with the Schmidt corrector?

texts: I want to fully understand how an SCT works. Thus, I figured out I can use some formulas that describe how an incident ray that is parallel with the axis would act after refracting off a Schmidt plate. I presume that the formulas would depend on the vertical and horizontal distances from the center of the plate. My goal now is to make a ray tracing model of an SCT with certain specifications. Please make your answer as simple as possible.I will be thankful if a nontechnical source is provided at the end of the answer.


texts: I'm aware for over 100 years the reason has been known how impact craters are almost always circular, but my question really is has there been any recent insights in the area? What other topologies are physically possible?


texts: What is faster than a supernova explosion?

texts:
Is a supernova explosion really fast or is it just fast, metaphorically speaking? Can you cite the fastest known phenomenon in astronomy (neutron stars, pulsars) besides supernovae in terms of explosions?

EDIT: yes, i mean the shockwave from supernova


texts: How do they know the newly-spotted gas near the center of our galaxy is molecular without knowing what gas it is?

texts:
Phys.org's Mystery gas discovered near center of Milky Way links to Teodoro et al 2020 in Nature Cold gas in the Milky Way’s nuclear wind who's abstract is shown below.
Question: My reading of the Phys.org summary and the abstract tells me that the gas is known to be fairly cold and molecular, and yet it is not known what the gas is! Is it so obviously hydrogen as H2 that nobody says this explicitly? If not, how does one determine a gas is molecular without knowing the chemical nature of the gas? It seems that if one resolves vibrational/rotational features in optical, IR or radio spectra one could simultaneously identify the species, but perhaps I'm naive. (Okay, I'm almost certainly naive!)
--
The abstract from Nature:

The centre of the Milky Way hosts several high-energy processes that have strongly affected the inner regions of our Galaxy. Activity from the super-massive black hole at the Galactic Centre, which is coincident with the radio source Sagittarius A*, and stellar feedback from the inner molecular ring1 expel matter and energy from the disk in the form of a galactic wind. Multiphase gas has been observed within this outflow, including hot highly ionized (temperatures of about 106 kelvin), warm ionized (104 to 105 kelvin) and cool atomic (103 to 104 kelvin) gas. However, so far there has been no evidence of the cold dense molecular phase (10 to 100 kelvin). Here we report observations of molecular gas outflowing from the centre of our Galaxy. This cold material is associated with atomic hydrogen clouds travelling in the nuclear wind. The morphology and the kinematics of the molecular gas, resolved on a scale of about one parsec, indicate that these clouds are mixing with the warmer medium and are possibly being disrupted. The data also suggest that the mass of the molecular gas outflow is not negligible and could affect the rate of star formation in the central regions of the Galaxy. The presence of this cold, dense and high-velocity gas is puzzling, because neither Sagittarius A* at its current level of activity nor star formation in the inner Galaxy seems to be a viable source for this material.


texts: Why didn't it seem darker outside with 66% of the sun covered by the moon?

texts:
I live in Austin, TX where the eclipse on August 21, 2017 reached its maximum point with ~66% of the sun covered by the moon.

Why didn't it seem any darker than a normal time with 0% eclipse?


texts: Is it safe to view moon at daytime through telescope

texts: I have a little 76mm Newtonian Reflector. I want to see the moon at daytime. Is it safe to look at sky at daytime when sun is shinning or there is any risk of sun.


texts: Why can space telescopes see through a planetary nebula?

texts:
I recently read the book "An Introduction to Planetary Nebulae" by Jason J. Nishiyama. Although I'm not an astronomy student, I could at least understand the written texts and less the physical formulas. So, I got a nice basis knowledge now -- or that's what I like to think.
Now, I browsed through some pictures of planetary nebulae. Mostly from Hubble and such. I know that they synthesize their images by combining infrared + visible light + uv-light, but I still can't comprehend why these telescopes are able to see INSIDE a nebulae when the shape of mentioned nebulae are spherical? For example, the planetary nebula: NGC 3242 (Ghost of Jupiter); NGC 7293 (Helix Nebula); NGC 6543 (Cat's Eye).

You can see the exposed white dwarf. And it looks like a thin shell envelopes that star, while the space between the star and the shell is scarce, like see-through. But wouldn't that image imply that the planetary nebula is disk-shaped? If the nebulae is spherical, then shouldn't there be "more nebulae" between the star and the shell? Or shouldn't the inner space be tinted in the color of the nebulae, because we look at a white dwarf that's behind a shell? I've drawn my problem with Paint too, sry for my artistic skills: Pattern A is what I thought it should be; Pattern B is what I think it looks like.


texts: I brought a new Astromaster 130 EQ and collimated it using a laser. However, whenever I try to observe moon I get a perfect picture but with other planets I see only a bright glowing ball which increases and decreases in size as I adjust the focusing knob. Also, I see secondary mirror shadow in between that bright glowing ball. Kindly help me where I have gone wrong. I am using all kinds of eyepieces from 20mm to 06mm. I have a Barlow lens 2x also.


texts: Relationship between precession and dating ancient alignments

texts:
Perusing the topics here. I am convinced that this is the right place to get some answers. Although it does appear that my question should be relegated to the "childrens table".

I am curious to understand more about the topic in archeoastronomy that is used to date ancient alignments utilizing calculations based on precession (which I very basically understand as an imaginary circle drawn in space by the earth's axis-line as the earth wobbles).

For example a "temple" in Peru has been dated as being aligned with the summer SOLSTICE(es)[edited] as of 6000 years ago by unnamed experts in the field. Other unnamed experts date it at 28000 years old - to which the first set of "experts" grudgingly concede is "remotely" possible - with an absolute qualification that there was no one here to build it that far back.(Statements of absolute certainty like this and, without proof make me itchy)

First part of question is: Why the huge discrepancy in the dating? Isn't the concept of precession a proven, quantifiable fact? (OK, that was two questions.)

Second part of question is: How do the first set of experts find themselves entrenched in their dating if they must also grudgingly agree that the second dating is remotely possible? (Both groups agree that neither the Inca nor any known predecessors actually built it.)

Anything which can help me to come to grips with this dating rationale would be great.

I am not versed in astronomy whatsoever. I can barely find Sirius on a clear night. And, math is also a foreign concept as well.
What I DO understand is Ancient Civilizations.
So, I would like to understand this topic more so that I can discuss it with others in the field - without sounding like a complete idiot.

[Edit to clarify before more protests stack up] I acknowledge that I did not recall and therefore omitted the fact that non-solar stellar alignments AT THE TIME of summer solstice are the basis for this alignment-dating. At least that is the theory I am discussing. (There are others) One window frames the Sun at sunrise while another frames some star. The third window... I cannot locate a theory of what it was supposed to be for. I just wanted to understand the basis for DRIFT over time. And, the accepted answer does that very nicely. Thank you everyone


texts: Why type Ia supernova reaches its peak absolute magnitude(~ -19.3mag) at around 15-20 days after the explosion?

texts: I'm not sure why type Ia supernova reaches its peak magnitude 15-20 days after the explosion. Is this luminosity generated only by the radioactive decay of nickel-56 to cobalt-56?(considering a half-life, I don't think the decay to iron-56 can be involved with it) Or is there any combination that produces the peak luminosity?


texts: Calculating the position angle of the moon (the rolling back and forth)

texts:
I have been watching the the NASA Moon Phases Video of the year 2020. Pretty much everything makes sense and I know how to calculate it including the moon phases, declination, azimuth, apogee and perigee except one thing, the position angle of the moon. Its donated by Pos. Angle at the bottom right of that video.

Apparently when the Pos. angle is 0, that means when the azimuth of the moon reaches 180 looking from the northern hemisphere, the phase's north direction of the moon is going to be perfectly aligned with the north direction on earth(180 azimuth):

Given the moon and sun current declination, How do you calculated the Position angle of the phase as it's described in NASA Moon phase 2020 ?
I'm looking for an output that matches NASA's calculations or close to it at any given date.
Any help would be much appreciate..


texts: How to calculate magnitudes in photometry?

texts:
I have a passband filter $b$ and a spectrum (SED) $f_{\lambda}(\lambda)$. Often the formula I find to get the flux in that filter $F_b$ is
$$F_{b}=\dfrac{\int f_{\lambda}(\lambda) T_{b}(\lambda)d\lambda}{\int T_{b}(\lambda)d\lambda}\,,$$
where $T_b$ is the filter response in band $b$, which is similar to the filters shown in figure 2 of The PLAsTiCC team et al (2018).

However, when I try to compute a apparent magnitude, I get the formula
$$m_b=-2.5\log_{10}\left(\dfrac{\int f_{\lambda}(\lambda)\,\lambda\,T_{b}(\lambda)d\lambda}{\int T_{b}(\lambda)\,\lambda\,d\lambda}\right)\,.$$

I don't understand from where does the $\lambda$ in the second formula arises, since I thought that
$$m_b=-2.5\log_{10}\left(\dfrac{F_b}{F_{\mathrm{reference}}}\right)\,.$$

For example, Casagrande & VandenBerg (2014) show both expressions in equations 1 and 3. However Bessell & Murphy (2012) (eq. 2) and Hogg, Baldry, Blanton, & Eisenstein (2002) (eq. 4) only show the second approach. Additionally, Hogg (2000) shows the K-correction without the added $\lambda$'s:
$$K=-2.5\log_{10}\left(\dfrac{1}{(1+z)}\dfrac{L_{\lambda/(1+z)}}{L_{\lambda}}\right)\,.$$

I don't know how to reconcile these different views. Can you help me?


texts: Was the whole Universe close to Big Bang very small, or just very dense?

texts:
I am sorry this question is probably silly for professional astronomers, of which I am not one.

I often hear at lectures that immediately after the Big Bang, the universe was small, say, the size of grapefruit or something like that. But because of inflation which stretched space at super-light (but finite), speed, the observable universe now, may not be the whole universe, and the whole universe may be infinite. If it is infinite, then it would seem that it had to be infinite in infancy as well, just very dense. In fact, it seems "in the limit", it had to be infinite even at Big Bang.

So, questions:

Was early universe small, or only dense but still infinite?

How about at Big Bang, was it infinite?

When astronomers say early universe was small, do they simply mean "the part of the universe which corresponds to our observable universe, was small"?


texts: Measuring parallax angle, same local time or the same time

texts:
I was thinking about measure the distance to the moon using parallax angle. But I have a confusion. I have read couple of articles about that, but one thing is unclear. Should we take pictures of moon at the same (literally the same, for example, taking both pictures at UTC 16) moment or should we take pictures the same local time (for example, if the timezone difference is 2h then the time difference between two taken pictures should be 2h).
So which one is the correct method for it? I think it is the former one, but I cannot tell why it is not the latter.


texts: Requesting book references for a non-expert person with math background or just a non-expert person

texts:
I've just finished my masters degree (to be exact previous week ^_^). And I'm completing my collection of applications of my studies. During previous year I started reading about Robotics and Coding theory (but in free times as a hobby). I knew that my field of study has application in Astrophysics but I didn't have enough time to go after this topic as well. But my main question; these days I felt into mood of having a general knowledge in Astronomy etc. everything related to stars and space.

As I'm not seeking the application of my field of study at astrophysics (if I were it was better to put this question at math stack instead of hear) and certainly almost all members here are not familiar with pure mathematics, I will be pleased if someone here introduce one or several nice texts or books which satisfy one of the two following set of conditions:
1- Some elementary sources which people who doesn't have any background can read them. The notes be exciting and the reader wants to continue it until its end.
2- Sources which also use mathematics (specially if the level of the math won't be only some calculus) but the dominant text be about stars and space.

Assume you are suggesting a text to a person as his first thing he will read in this topic. And if you think a series or a number of texts are suitable, please give an order to which should be read first and then which for the next and so on.
(Also please note that I don't like to read guesses! So texts contains we guessed x should be because of y or we think the world is ... are not interesting to me)

Thank you for your attention.


texts:
Does our solar system have enough bodies in orbit to appear to twinkle to another distant solar system viewed from the side?

https://en.wikipedia.org/wiki/List_of_asteroid-discovering_observatories

https://phys.org/news/2018-11-nasa-lucy-sky-asteroids.html


texts:
Wolfram Alpha's query: Sun Earth distance in AU gives:

  1.012 au  (astronomical units)

They definitely use this value in other places too - e.g. Earth-Sun $L_1$ distances from Earth and from Sun don't add up to 1 AU - they are 0.01009 AU and 1.001 AU respectively.

Contrarily, Google gives Earth Sun distance in AU = 1.000 AU.

The top search results for Sun Earth distance 1.012 AU yield a page with a big heading of "Paranormal UFO Aliens Wow!".

Is the 1.012 AU value an error in Wolfram Alpha, or is it some little-known adjustment, redefinition or other quirk of the unit or the Solar System?


texts: If our galaxy is mostly empty space then why does it look so solid?

texts:
This question has probably been asked quite a few times, but I'm having trouble finding anything due to the fact that this question is very specific in nature. So I'm asking again.
We know that outer space is mostly empty and stars are very far apart from each other. So much so that when two galaxies collide, the stars within them will just fly past each other instead of crashing. But when I look at pictures of the Milky Way, (some of them are real pictures of the center of the Milky Way taken from earth, while others are renderings) it looks so bright and so solid, like theres so much stuff in there. It becomes hard to believe how it can be that it's mostly empty.
Can anyone please explain what's going in there?
Thanks.


texts: The destruction of the Universe by a bubble

texts:
So I was watching a documentary on television about this and a number of theories that are proposed as to what the fate of the Universe will be:

"Big Crunch"
"Big Freeze"
"Big Rip"

I understand these, but there was also another scenario which I don't understand:

A bubble forms in the Universe and expands at the speed of light. It annihilates everything in the Universe that it comes into contact with, until eventually at a very, very distant time the Universe ceases to exist. Does anyone have any clue as to what they were talking about (because I don't)?


texts: How can I work out longitude and latitude through my telescope?

texts:
We purchased a Celestron Astromaster 90EQ Refractor Telescope and a Celestron Powerseeker Accessory Kit about four years ago but have not used it much for many reasons. As such my husband and I are complete beginners and was hoping for some advice please?
My Dad passed away during the lockdown and we have had a star named after him.
It's a "UK" based star and we will be sent longitude and latitude co-ordinates enabling us to search for it.
On Christmas day we are hoping to use the telescope, with our children, to point out my Dad/Grandad in the sky. However, my husband has now said he thinks you have to buy an accessory or something to do this and the telescope and accessory pack we have does not have the necessary equipment ?.
Can anyone advise how you can do this please and, if we need to purchase an accessory, recommendations?
Thank you very much for any help in advance.
Sue Davis


texts: Why don't we see more supernovae in our galaxy?

texts:
I found this on the Wikipedia page on supernovae:

  The total supernova rate in our galaxy is estimated to be about 4.6 per century, or one every 22 years, although we haven't actually observed one for several centuries.

That sounds like a freak occurrence to me.

It made me curious about the numbers so I refreshed my knowledge on the Poisson distribution. It is not specified how many centuries we have not seen a supernova, but let's assume it's only two.

If on average 9.2 supernovae happen per 2 centuries then the chance of having exactly 9 in any two century period can be calculated with:

$$p(k) = {\lambda^k \times e^{-\lambda} \over k!}$$

Filling in $k=9$,

$$p(9) = {9.2^9 \times e^{-9.2} \over 9!}=0.1315$$

The chance that there is not a single supernova in a two century period is:

$$p(0) = {9.2^0 \times e^{-9.2} \over 0!}=0.00010104$$

That's about a 1 in 10,000 chance. If "several" means 3 centuries, then the chance of not having any supernova is as little as 1 in 100,000.

So from this I can only come to the conclusion that supernovae did in fact happen in the Milky Way in the last two centuries, but that we didn't see any of them.

But now I'm confused. I read about supernovae that have happened in galaxies billions of light-years from here that lit up as the brightest source of light in the sky for days. Surely we would be able to see all of the supernovae that happen in our Milky Way (no more than 100,000 light years away) with the naked eye, let alone with telescopes?

Are some supernovae so much more super than others? Are all of the Milky Way's supernovae visible to the naked eye?


texts:
Iapetus's hemisphere facing Saturn is dark, whereas the opposite one is bright.

Could a cryo-volcano be the reason behind this colour difference in Iapetus's hemispheres?


texts: Will NEOWISE be known as the Great Comet of 2020?

texts: Many or all comets visible to the naked eye are referred to as Great Comets of their year(s). Might NEOWISE too be called like this? Why or why not? Is the IAU responsible for declaration of Great Comets or another organization or no such body?


texts: Has the Kepler data been examined for light curves due to phase changes of non-transitting hot Jupiters and other?

texts:
I found this article http://www.epj-conferences.org/articles/epjconf/pdf/2011/01/epjconf_ohp2010_03005.pdf from 2011 announcing that two dozen secondary eclipses of "hot Jupters" have been seen in Kepler data. I even think I have seen a light curve showing phase changes of a transiting planet. This makes me think that there should be evidence in the Kepler data for non-transiting planets since the amplitude of the light curve due to planetary phase changes shouldn't be much different from that of a secondary eclipse as long as there is a fairly high inclination (say over 60 degrees).

Is anybody aware of such an examination of the data?


texts: Hubble Space Telescope (HST) observing modes

texts:
I have been reading some literature on using the HST to take spectra of exoplanet atmospheres when exoplanets transit in front of their stars. The literature mention two modes of observations, 'spatial scanning' and 'staring', in order to obtain spectra of the atmospheres of these exoplanets.

Can anyone explain conceptually what the difference is between these two modes?


texts: What's a good model for the vernal equinox year? Can it increase forever?

texts:
Duncan Steel (2002) makes an interesting argument that the notion of year is (1) somewhat arbitrary and, more interestingly (2) one notion of year can increase while another decreases.

His examples are tropical year (which decreases over time, at least in the linear Newcomb model) and the vernal equinox year which seems to be currently increasing.

Actually looking at the data that his point is based on, namely Meeus and Savoie (1992), we can find another measure that is decreasing, namely the solstice year(s), either June or December; the autumnal equinox year is also decreasing:

Steel actually uses the fact that the  vernal equinox year is increasing as an argument that the Gregorian calendar is "getting better" in a sense that it's closer to hitting its target year length. (The opposite is the case if one consider say either of the solstice years... or the tropical year, which is the point that a lot of other people make, e.g. Moyer (1982).)

What I really want to ask here: assuming a simple linear decrease model for the tropical year (e.g. Newcomb), will the trends of the equinox and solstice years reverse at some point in the future?

(And if it's not too much extra to ask:) What are the implications of the more complex models for the tropical year (e.g. cubic VSOP) on the evolution of the equinox and solstice years?


texts: Can all areas on Earth experience a total solar eclipse?

texts:
A solar eclipse only puts part of Earth at a time in shadow, so I'm not talking about a solar eclipse that casts a shadow over all of Earth.

What I mean is: are there any areas on Earth where, no matter how Sol, Earth and Luna are aligned, a total solar eclipse is impossible? Can the penumbra cast a shade on any part of Earth, or are there certain areas where a penumbra due to a solar eclipse cannot occur?


texts: Relationship between absolute magnitude of a star and its luminosity?

texts:
Why does this relationship involves the absolute magnitude of the sun and its luminosity?
How to derive an expression relating the absolute magnitude of a star to its luminosity?

So according to the magnitude equation, $$m - M = 2.5\log\left(\frac{d^2}{d_0^2}\right)$$
$$\Rightarrow M = m - 2.5\log\left(\frac{d^2}{d_0^2}\right)\ \ (eq1)$$

and Luminosity is $$L = 4\pi(d^2) \times f$$
 $$\Rightarrow d^2 = \frac{L}{4\pi \times f}\ \ (eq2)$$

Plugging eq2 to eq1 would have seem reasonable, but how is absolute magnitude of the sun and its luminosity used?


texts: Why are there no stars on New Horizons images of Pluto

texts:
I followed the New Horizons Mission a little, and saw among others this image of Pluto:

I wonder, why you can't see any stars on it. As far as my very basic knowledge in astronomy goes, I think you can even see stars during a "Sun-Moon-Eclipse" here on earth. So besides having here a spectacular "Sun-Pluto-Eclipse", I further more ask myself if that eclipse is even necessary to see other stars, as those pictures are already taken quit far away from sun, and outside an atmosphere.

Is that not enough to see stars? Or does NASA remove the stars from the images, before publishing them? (If so, why? Not to distract?)

Thanks for answers!


texts: How can a gas planet become tidally locked with a star?

texts:
Supposing a planet is made entirely of gas, is it possible for the planet to become tidally locked with the star it orbits?

I know when a terrestrial planet orbits a star in a different amount of time than the length of its day, the planet warps continuously, resulting in tidal heating. This heating takes energy from the planet's rotation until it stabilizes in a tide-locked formation.

I believe a purely gaseous planet would also have some sort of internal heating for the same reason. But because the "surface" of a gas planet is fluid, it must also obey thermodynamic principles.

As a basic example, the side of the planet facing a star gets hot, and the side facing away gets cold. The hot gas expands and the cold gas contracts and you get convection, Hadley cycles, prevailing winds, and chaotic weather patterns.

So it seems like there's a battle between gravity, which wants the gas to stop moving and become tide-locked, and thermodynamics, which wants the surface to roil with convection.

Does one process dominate over the other? Does one process slowly take over as the other fades away? Can it go either way?


texts: What is the color of Venus if it has no atmosphere?

texts: Mars looks reddish and Mercury looks gray because they lack thick atmospheres and we can see their "real color" easily, how about Venus? Is the surface of Venus really yellowish in color?


texts: Why doesn't the IAU consider Quaoar to be a dwarf planet?

texts:
Only Ceres, Pluto, Haumea, Makemake and Eris are listed by the IAU as dwarf planets. Quaoar is bigger than Ceres, yet it doesn't make the cut. In fact, it fits the IAU description of a dwarf planet

  The shape of objects with mass above $5 \times 10^{20}$ kg and diameter greater than 800 km would normally be determined by self-gravity

since Quaoar has a mass of $1.4 \times 10^{21} $ kg and a diameter of 1110 km.

Futhermore, light-curve-amplitude analysis shows that Quaoar is indeed a spheroid.

Why isn't Quaoar considered to be a dwarf planet by the IAU ?


texts: Why doesn't Jupiter increase the chances of an asteroid to strike the Earth?

texts:
I often hear the hypothesis that Jupiter decreases the chances of an asteroid strike on the Earth, but my question is, why does it only decrease the chances ?

While Jupiter can deflect some asteroids that were originally going to hit the Earth, it can also deflect some asteroids towards the Earth. How can we be sure that Jupiter only decreases the chances of an asteroid impact on Earth ?


texts: Is lithium considered a metal in astronomy?

texts:
My lecture notes are conflicting on this issue so I seek definitive clarification. In Astronomy, are metals either
A: the elements which form within stars (i.e., not Big Bang nucleosynthesis, where most of the lithium (Li) was formed as far as I know)
or
B:  everything heavier than helium (He)?


texts: Does the sun blow a bubble into the interstellar medium?

texts:
I once read that the solar wind is more or less blowing a "bubble" into the interstellar medium. However, the numbers I found while trying to verify this don't give me a clear picture yet:

The interplanetary medium seems to have an average density of 5 particles per cm³ (at least near earth).
The interstellar medium density varies widely, from 10⁻⁴ to 10⁶ particles per cm³.

What I did not find was the density of the interstellar medium directly around the solar system. I know that the solar wind is interacting with the interstellar medium, which is responsible for the termination shock, but I'm wondering if the density difference is large enough that one can say that the sun is actually blowing a "bubble" into the interstellar medium.

So, is the density inside the heliosphere much less than directly outside of it?


texts: Did the Moon ever approximate a geosynchronous orbit above the early Earth?

texts:
Simulations suggest the early Moon formed about 20,000 to 30,000 km away from Earth which is less than the current geosynchronous orbit of 42,164 km.  The Moon is now 384,000 km away from Earth.  Does that mean the moon approximated a geosynchronous orbit for a time on its journey outward to its current orbit?  I recognize because of the Earth's tilt of 6.7 degrees the Moon does not orbit around the Earth's equator and therefore a geostationary orbit wasn't possible.

If the early Earth was a water world covered almost entirely with water at the time it formed, what would happen when the Moon approached geosynchronous orbit and applied its much more amplified gravitational force upon the Earth?  Would there be tidal bulges of water at both ends exposing land at the polar regions or would the water eventually make its way toward the Moon side possibly exposing or making a bulge of land at the opposite end of the Earth?


texts: What method is used to calculate the 'quality' of a solar/planetary image?

texts:
What sort of algorithm is used to calculate the relative quality of an image, such as is performed by PIPP, Registax, or Avistack?

Such applications take planetary or solar images and prior to stacking and sharpening, they can optionally select the best images. When looking at images to see which have been selected by the algorithms, I sometimes see images that I wouldn't have selected. This has piqued my curiosity as to what methodology is employed in ranking or grading the images. With a bit of a primer to understand the concepts, I would like to put some thought into how such algorithms might be improved...


texts:
When a rogue brown dwarf star hits a white dwarf star, will the collision at most cause a nova?
Or can the explosion be heavier?


texts: Common sense tells us a year is conveniently divided exactly into four seasons. I'm wondering whether that means the earth's tilting cycle is equal to its orbital cycle. If so, there must be a reason. Do we have a proof?


texts: If Jupiter and Saturn were considered stars, which of their moons would qualify as planets by 'clearing the neighbourhood'?

texts:
Jupiter has four and Saturn seven spherical satellites. Jupiter's moon Ganymede and Saturn's Titan are larger than Mercury and they're the most massive moons at 0.0248 M♁ (Ganymede) and 0.0225 M♁ (Titan). Do these moons 'shepherd' the other moons' and asteroidal satellites' orbits? Ganymede is three times the mass of Europa and quite close to it, so does it dominate Europa's orbit? Callisto on the other hand is twice as far as Ganymede and the 3rd most large and massive known moon while quite lonely in its orbit. Would it qualify for a planet too? If Jupiter was a star, I think Ganymede and Callisto would qualify as planets, Europa would be a 'dwarf planet' (because of being dominated by Ganymede's and perhaps Io's orbit) and Io I don't know, it probably would be more controversial.
Saturn's moon Titan is more massive than the other six spherical moons altogether, so I think it would clearly be a planet. The other moons (except Iapetus) are quite close to each other so I think they'd be considered 'dwarf planets' except Iapetus which is very far from Saturn and also quite lonely, so it might qualify as a planet (there might be an issue concerning its shape but my question is about the 'clearing the neighbourhood'/planetary discriminant point). Mimas would near-certainly be a 'dwarf planet' because it's about within the rings which alone are about 2/5 the mass of Mimas. So I think Titan and Iapetus would be considered 'planets', Mimas a 'dwarf planet' and the other spherical moons of Saturn probably too.
What would you say? Which Jovian and Saturnian moons would be considered planets and which ones 'dwarf planets' as per the proposed planetary discriminants if Jupiter and Saturn were stars?


texts: Description of Henyey tracks on wikipedia incorrect?

texts:
So, if you search for Henyey tracks on wikipedia (I know, the shame of it!) you will come across this statement:

  The Henyey track is characterized by a slow collapse in near hydrostatic equilibrium. They are approaching the main sequence almost horizontally in the Hertzsprung–Russell diagram (i.e. the luminosity remains almost constant).

Ignoring the bad grammar, I take umbrage with the following:

  the luminosity remains almost constant.

I would say that the apparent horizontal track associated with the Henyey track is more due to the fact that in a typical HR diagram the temperature axis ranges over about one order of magnitude, while the luminosity axis may vary over five or more orders of magnitude.

So, to frame this as a question, would you agree or disagree with the statement on the site or with what I have stated (i.e., that the Henyey track is an apparent horizontal track caused by the nuance in the magnitude scales of luminosity in comparison with the temperature)?


texts: Recognising lunar month only by looking the sky

texts: Suppose somebody went on an island and don't have sense about time, dates, months. The only thing he has is seeing the sky. Is it possible to recognise which lunar month it is by just looking the sky? I ask this because I know how to see beginning of the lunar month, but how to recognise which month is it? So we can look at the Sun, Moon, the stars. Is there a way to figure that out?


texts: Stellarium software with DE431 for eclipse 10000 yrs ago

texts: Hi I am a novice in the field of astronomy. I want to find out the dates for lunar and solar eclipse 9000-11000 yrs ago. I found out that Stellarium is based on VSOP87 which is recommended for 4000 yrs ago. However, the software allows one to use DE431 which can provide planet ephemerides upto 13000 yrs ago. I am wondering if I can calculate eclipses observed in India accurately using DE431.


texts: Some neutrinos are fermions. There are quite of few of them out there. Could their degeneracy pressure give rise to an expansion force to explain Dark Energy?


texts: Size of the Universe before the Big Bang

texts: I can't understand how all the matter contained in the Universe, was contained in an area the size of a grapefruit before the Big Bang ? How could so much matter have been compressed in such a tiny space ?


texts: Look up current positions and velocities of solar system objects

texts:
I am working on an orbital dynamics code and, for fun, I would like to model the Sun-Earth-Moon system with my code.  I can look up the masses of each object just fine, as well as average distances and relative velocities (which I can also calculate).  However, I would like to model the Sun-Earth-Moon system in great detail.  I would like to know the positions and velocities of each object on any given day (today works just fine) so that I can represent the orbits of these objects with great accuracy, hopefully even reproducing their actual ellipticity.  Where can I find values for:

current Sun-Earth distance
current Earth-Moon distance
current Sun-Moon angular separation
current velocity of Earth relative to Sun (or relative to Sun-Earth(-Moon?) center of mass)
current velocity of Moon relative to Earth

and, maybe even

current velocity of Sun relative to Sun-Earth(-Moon?) center of mass.

Is there any resource that provides these current values?


texts: Is it dangerous to look at an L-type brown dwarf from too close?

texts: Are some L-type brown dwarves bright enough so that it would be dangerous to the eyes if one looked at them from habitable distance or closer? If not, are the weakest M-type stars undangerous to look at from habitable distance too? T-type brown dwarves are dim enough that there shouldn't be any danger, and Y-type ones are invisible.


texts:
In Stephen Hawking's A Brief History Of Time, the book mentions that "in regions that were slightly denser than average...galaxies were born." (pg 123). However, according to many other sources, the hypothesized Population III stars lived and died first, giving birth to black holes that would cause the formation of galaxies. So I am wondering, which statement is correct?
Edit: Maybe a duplicate of this post?


texts:
So I live in a suburb in Victoria, Australia. Less than an hour away from the city and I guess there is a bit of light pollution because from my backyard I can probably only see about 15 - 20 stars (probably less), I'm wondering what these stars are, and what I'll see when I get this telescope:

https://www.opticscentral.com.au/saxon-707az2-refractor-telescope.html?___SID=U#.WXQNMtN940r

This is going to sound stupid but how do I know when there are planets in the sky that I can see? I don't think I've ever actually seen a planet other than the moon.

Thank you.


texts: Are binary neutron star mergers needed to explain the abundance of gold?

texts:
The NPR News item Astronomers Strike Gravitational Gold In Colliding Neutron Stars mentions and quotes "Daniel Kasen, a theoretical astrophysicist at the University of California, Berkeley:"

He spent late nights watching the data come in and says the colliding stars spewed out a big cloud of debris.
"That debris is strange stuff. It's gold and platinum, but it's mixed in with what you'd call just regular radioactive waste, and there's this big radioactive waste cloud that just starts mushrooming out from the merger site," Kasen says. "It starts out small, about the size of a small city, but it's moving so fast — a few tenths of the speed of light — that after a day it's a cloud the size of the solar system."
According to his estimates, this neutron star collision produced around 200 Earth masses of pure gold, and maybe 500 Earth masses of platinum. "It's a ridiculously huge amount on human scales," Kasen says. He personally has a platinum wedding ring and notes that "it's crazy to think that these things that seem very far out and kind of exotic actually impact the world and us in kind of intimate ways."

Has the merger of neutron star binaries been necessary to explain abundances of the heavy elements such as gold and platinum, or is this just an anecdotal item? How important are binary neutron stars for the abundances of heavy elements such as gold? Is there a particular or notable paper I can read on this?
I have already read this answer but I'm looking for a better explanation of the need for this kind of merger to explain abundances. I'm pretty sure there is nothing in any observed gamma ray events that shows spectral lines of gold or any identifiable heavy element (due to the incredible doppler broadening), so the connection must actually come from simulations.


texts: Cycles that are longer than the year, but less than 1000 years

texts:
Wondering what primary astronomical cycles there are that are greater than a year (which is a cycle around the sun), but less than 1000 years-ish (on that scale). There is the cycle of the moon phases too, and the cycle of the earth making a full rotation (a day). But I wonder if there are any cycles at a bigger-than-a-year scale, but not too big. I guess there's also the cycle of a stars life (but that is kind of a big scale, on the order of billions of years), but I am more wondering in terms of rotation what sort of cycles there are. I guess there is another one, Axial Precession, but that is a bit large such as on the order of tens of thousands of years. Maybe there are some cycles of the planets in which they are visible from earth, or of their moons, or of comets, or of nearby stars or that kind of thing.

If there were ones around each of these points that would be nice: 2, 5, 10, 20, 50, 100, 500, or 1000 years sort of thing.

Basically I am looking for "interesting" or "popular" cycles rather than just picking any of them (since every planet and moon has an orbital period). I am looking for ones that (a) we can observe most easily either directly without a telescope, or with a simple telescope/binoculars, and/or (b) have significance of some kind historically perhaps. If there is one that is historically significant or historically popular (say something Newton or Kepler talked about for example) but is difficult to see, that would be okay too.

For example, I vaguely remember hearing people mention "hey these x and y planets just aligned, it only happens every x years", so did a quick search and found this saying Jupiter and Mars align every 2-3 years. That is pretty interesting, since it is a small timeframe, bigger than a year, and you can see it pretty easily. I would like to find a few others though, spread over the scale (as mentioned above) from 1-1000 years. Here is another interesting one, a "Super Blood Blue Moon" as NASA is calling it, which hasn't happened since 1866 (so let's say ~150 years). Not sure if that will cycle every 150 years like that, but something that does cycle similar to this is also exactly the sort of "cycle" I am looking for.


texts: Why is Gaia operating around Earth orbit? Why not send it to Neptune's orbit?

texts:
Gaia is an astrometry spacecraft that's currently operating around the Sun-Earth L2 Lagrangian point. Question: why here? Why not the Sun-Neptune L2 Lagrangian point? By orbiting the Sun at a larger distance, it should be able to get more accurate parallax measurements.

Only reason I can think of is cost. I'm not familiar with estimating how expensive space probes cost, but Wikipedia says Gaia cost ~\$1 billion and this is comparable to the cost of the Voyager program, which also cost about ~\$1 billion. Of course Gaia's instruments should be more sophisticated than Voyager's, but there were also two Voyager probes, not one.


texts: Why do we have so few neutrinos to study?

texts:
According to Wikipedia most neutrinos passing through the Earth emanate from the Sun. About 65 billion (6.5^10) solar neutrinos per second pass through every square centimeter perpendicular to the direction of the Sun in the region of the Earth.

However the IceCube Neutrino Observatory or other neutrino detectors only detect a few neutrinos every year. Why?


texts: Is this a potential planetary setup for the Alpha Centauri System?

texts:
I originally posted this on the World Building SE, but I was suggested to post it here for a better answer.
I am attempting to create a [semi]plausible star system in Alpha Centauri for a series. The system was originally intended to have 1 habitable planet per star, but after my first couple of attempts I noticed that it might be possible to get at least 3 planets to be habitable. The latest attempt actually shows 5 potentially habitable planets (2 of which are a binary[That is another SE question]). Ultimately, I would like this to be the case as it allows a much more diverse universe for the series.

Attempt:

aCen A Planets

aCen B Planets

I don't have access to, or knowledge how to use, any form of software like Universe Sandbox. I have read several reports on planet orbits and I think I did an "ok" job with this.

Here are some papers that I have looked at for this:
http://adsabs.harvard.edu/full/1997AJ....113.1445W
https://arxiv.org/pdf/1801.06131
https://core.ac.uk/download/pdf/25201586.pdf

My main concern is that the planets might orbit too closely. I have attempted to find a formula (that I could understand) that could aid in spacing the planets. The nearest I could manage is using the Mutual Hill Radii. There are conflicting reports where one says that 10 - 12 MHR (Delta-H) is good or a tightly packed system. Earth and Venus have around a 25 MHR value. One of the reports that I linked mentioned up to 25 MHR for aCenA, but it also shows several other numbers and after trying to comprehend everything my brain reached orbital velocity.

Question: Is the planetary spacing stable enough to host planets on Gigayear timescales?
They don't necessarily need to be able to have spawned life, but they should be able to support life with little to no human intervention.

Note and Bonus, aCen V is a binary planet that orbits with a Semi-Major axis of 750,589km with an eccentricity of 0.01204. (There is an error in the image in Yellow that shows 148.623 and Eccentricity of 0.0910). The inclination should be 0 as both planets should be on the same plane.
Last note: The Semi-Major Axis distance is in Megameters (1 million meters).


texts: Eccentric Earth Orbit and Day/Night Cycles

texts:
If the Earth's rotational axis was perpendicular to the orbital plane, but the orbit also highly eccentric (sun at center), how would that affect the day/night cycles?

What I think is that during the time the Earth's closer to the sun, because the gravitational pull is stronger, it would make the Earth spin faster than thereby creating much shorter days than if the Earth was near the focii of the orbit (on which the days/nights would be longer). Is this line of thinking correct?


texts:
I need help identifying a blob.

Last night I took a picture of Polaris, and there is a weird blob in it that I can't identify.  Star charts and other pictures on the 'net do not show this blob.  I took three pictures (with slightly different positions) and the blob has the same shape and position (relative to Polaris) in each.  The blob does not exist in other pictures that I took (pointing in completely different directions).

Taken with a Canon t5i, 300mm lens, 3 minute exposure on a motorized equatorial mount.  Viewing conditions were terrible, with lots-o-moon and smoke from forest fires.  From Colorado.

Polaris is the brightest star, slightly left and down from center. Yildun, the next star in the handle of the little dipper, is near the upper left corner. The blob in question is very near, and slightly up and to the left of Polaris.

Edit:

Simple Lens Flare:  I don't think it's lens flare (LF).  With LF, you can draw a straight line through the photo where the flare, the bright object, and the center of the photo all line up. Also, the LF tends to match the shape of the bright object.  I don't think it's LF because the blob doesn't line up with the center of the photo and a bright object, and there are no bright objects that have the same shape as the blob (with that weird tail to it).  Also, I have three photos with the same blob. In each case, Polaris (the brightest point in the photo) moves slightly because the camera moved slightly but the blob moves exactly with Polaris and not somehow relative with the center of the frame.

Under careful analysis, there IS lens flare, but it is where you'd expect it and not where the blob is. If you draw a line though Polaris and the center of the frame, there is a flare on the opposite side of the center and the same distance away from the center as Polaris is.

Schmutz in the camera:  I don't think it's dust/dirt in the camera/sensor.  If that were the case then I would have seen it in other photos from the night, and I didn't.  Also, the blob would have stayed in the same position in the frame and not moved with Polaris as it did in the 3 different photos that I have of it.

Not-So-Simple Lens Flare:  Maybe.  Although I cant imagine how this would happen unless some lens elements were not radially-symmetric and/or elements were not lined up on a common axis.  But I don't know everything about camera design so maybe this is possible?

There's a new blob in the sky:  Possible.  Although I think the odds of "user error" are much higher.  Maybe someone else wants to take a new photo of Polaris?  (wishful thinking)


texts: Is dust the only reason why the Martian sky is so bright?

texts: The atmospheric pressure on the surface of Mars is 0.00609 atm on average and 0.012 atm at most in the Hellas Basin. On Earth, at these pressures, which are found at altitudes about 30-35 km (19-22 mi), the sky is very black and full of stars. On the Martian surface however the sky is still bright (namely orange) and no stars/planets are visible in daytime from what is considered the sea level (except the Sun and maybe the Earth). Is this due to the many dust in the Martian atmosphere only or are there other reasons too for why the sky is not black but still orange at these pressures?


texts: How to calculate the luminosity of a quasar?

texts:
Is there a relation between the distance ($d$) and the apparent bolometric magnitude ($m_{bol}$) of a quasar and its luminosity ($L$) in watt? If so, what is that relation?
Thanks in advance!


texts:
As the Moon is tidally locked to Earth, that means, from Earth we can only ever see one side of the moon.

Can we see all the sides of the Earth, from the moon?


texts: How to understand the meaning of this International Occultation Timing Association (IOTA) lunar occultation of Mars map?

texts:
Earthsky.org's Watch Mars disappear behind the moon on February 18 as well as Moon occults Mars morning of February 18 include the image below and credit it to The International Occultation Timing Association or IOTA.

But I can't understand the meanings of the various colored lines on the map. There's the big blue droopy infinity sign, and attaching to both ends an upward-facing hyperbola with three colored segments.

I went to https://occultations.org/observing/occultation-predictions/ and clicked the Lunar Occultations link, but it only returns error 404 file not found.

Is it possible for someone to explain the meanings of these lines?


texts: Based on discoveries of new TNOs since 2016, is planet Nine still necessary to explain the TNO's orbits?

texts:
It is almost five years since the announcement of the likelyhood of another planet that is responsible for clustering eTNO's orbits, very close perihelia of detached dwarf planets to the Sun, and high inclinations of TNO's orbits. Since then there have been 316 new TNO discoveries by the Dark Energy Survey and some claim the observed clusterings are due to observational bias and that the DES disproved the necessity of planet Nine. Others still claim only a planet Nine seems to explain the observed orbits. Who is right?
Either the existence of planet Nine was really concluded from observational bias or there is indeed a likely mass that is responsible. Is Planet Nine still the most reasonable explanation for the orbital parameters of the 14 TNOs in question?